Cardiovascular disease is a leading cause of morbidity and mortality. Evaluation of vascular disease is a common indication of radiological examinations. Vascular imaging was first achieved through the use of conventional x-ray angiography. With the development of advanced imaging hardware and techniques in the past decades, the expectation for vascular imaging is not limited to visualization of stenosis or occlusion of blood vessel. Information concerning circulation or dynamic flow of blood to a particular anatomical region becomes more important for the diagnosis of diseases such as arterio-venous malformation (AVM) in the brain. X-ray angiography is currently widely used for diagnosis of various vascular pathologies and provides two-dimensional projection images of blood vessels with high spatial and temporal resolution. X-ray angiography however, is invasive and requires arterial catheterization. The formation of imaging contrast relies on administration of an iodinated contrast agent which may induce nephrotoxicity and further complications. Besides that, two-dimensional projection images make it difficult to visualize pathology from multiple viewpoints.
Computed tomography (CT) provides high spatial resolution 3D (three dimensional) image data sets over a short time duration. Oblique multiplanar reconstruction of the 3D image data sets is used to assess a vascular structure. However, temporal resolution of high spatial resolution CT 3D images is relatively poor and CT examinations typically require a relatively high radiation dose. Furthermore, use of a contrast agent is necessitated to generate adequate imaging contrast. Duplex ultrasonography is also used to provide an inexpensive and least harmful means for studying vascular structure. However, it is highly patient and operator dependent and the results are difficult to duplicate.
Magnetic resonance imaging (MRI) is a modality commonly used in routine clinical applications and provides good soft tissue contrast to differentiate normal and pathological structures. Imaging orientation and imaging contrast are freely manipulated by adjusting the imaging protocol and parameters. Neither ionizing radiation nor iodinated contrast agents are involved and MR images provide anatomical and functional information from the same imaging session. Contrast-enhanced magnetic resonance angiography (CE-MRA) is used in routine evaluation of vascular disease. Typically a Gadolinium-based contrast agent is intravenously injected. The shortening of blood longitudinal relaxation time (T1) from such a paramagnetic agent yields enhanced blood signal intensity in resulting images. With appropriate setting of imaging parameters, high spatial resolution vascular images are acquired. A limitation of this technique is that, functional information is not well depicted due to relatively low temporal resolution (on the order of seconds) of acquired 3D image data compared to flow velocity of blood. Further, Nephrogenic Systemic Fibrosis (NSF), a systemic disorder potentially inducing devastating or even fatal functional consequences has been associated with Gadolinium-containing magnetic resonance contrast agents especially in patients with impaired renal function.
One known non-contrast agent blood flow imaging system discussed in a paper dated June 1995 by M Essig et al. entitled “Cerebral Arteriovenous Malformations: Improved Nidus Demarcation by Means of Dynamic Tagging MR-Angiography” employs a spatially localized tagging system. The known system limits tagged blood inflow to a ROI (region of interest) to flow from a spatially localized region associated with tagging pulse width and also only provides a 2D (two dimensional) imaging output. The known system also fails to provide comprehensive signal quality and speed characteristics desirable in this type of MR imaging. A system according to invention principles addresses these deficiencies and related problems.